The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to, a pulse sequence, method, and apparatus for multi-slice acquisition using fast spin echo imaging to acquire black blood contrast images.
MRI uses radio frequency pulses and magnetic field gradients applied to a subject in a strong homogenous magnetic field to produce viewable images. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Current techniques for the acquisition of multi-slice fast spin echo (FSE) images require that each slice be imaged in separate breath-holds in order to attain black blood contrast in a gated sequence. Such conventional gated-FSE acquisitions are able to acquire images from only one spatial location per breath-held acquisition because the second inversion recovery RF pulse is slice selective only over the imaged slice.
It would therefore be desirable to have a technique to acquire black blood contrast images using fast spin echo acquisitions with improved multi-slice acquisition for efficient imaging that is capable of imaging across one or more Rxe2x80x94R intervals.
The present invention relates to an MR pulse sequence, apparatus, and a technique for efficient multi-slice acquisition with black blood contrast in fast spin echo imaging that solves the aforementioned problems.
The proposed technique includes a non-selective inversion RF pulse, followed by a broad-band slice selective pulse that re-inverts the spins in the slab encompassing the slices to be imaged. After an inversion time, RF excitation pulses with a fast spin echo readout are executed acquiring data for each spatial slice in an order that provides optimal blood suppression. The inversion time is preferably selected such that the blood signal is close to the null point. This technique differs from conventional gated fast spin echo imaging with black blood image contrast in that the re-inversion or tip-up pulse in the conventional technique is effective only in a single slice of interest which results in only one slice or spatial section acquired per breath-hold or acquisition. The present invention allows for multi-slice acquisition.
In accordance with one aspect of the invention, a method of multi-slice fast spin echo image acquisition with black blood contrast is disclosed that includes a non-selective inversion pulse and applying a re-inversion pulse that is slice selective over a region encompassing a plurality of slice selections. The method includes timing execution of the series of RF excitation pulses with fast spin echo readout such that signal from black blood is near a null point. Data is then acquired for each spatial slice.
In accordance with another aspect of the invention, a computer program is disclosed for multi-slice coverage in a single acquisition with black blood T2- weighted image contrast. The computer program has a set of instructions that when executed by a computer cause a computer to generate and cause application of a non-selective inversion RF pulse to a slab of slices, each having a predefined thickness. The computer program also causes the computer to generate and cause application of a slice selective re-inversion RF pulse having a slice thickness greater than the predefined thickness of a single slice and apply an inversion time so that a null point of blood within the slab occurs in a middle of an acquisition. A series of RF excitation pulses is applied and MR data is acquired for each slice in the slab.
In accordance with another aspect of the invention, an MR apparatus to produce consistent contrast in FSE image acquisition is disclosed. The apparatus includes an MRI system having a number of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system and an RF modulator controlled by a pulse control module to transmit RF signals to an RF coil assembly to acquire MR images. The MRI apparatus also includes a computer programmed to apply a pulse sequence having a non-selective inversion pulse to invert spins in a longitudinal direction across an entire slab of slices and a slice selective re-inversion pulse having an implied width at least as large as that of the non-selective inversion pulse. The pulse sequence applied by the computer also has a series of excitation pulses having fast spin echo readout spaced apart from the slice selective re-inversion pulse by an inversion time to acquire data for each slice in the slab.
In accordance with yet another aspect of the invention, a pulse sequence for use in multi-slice MR data acquisition is disclosed. The pulse sequence includes a non-selective inversion pulse applicable to a slab of slices and a slice selective re-inversion pulse applicable to at least a number of the slices in the slab of slices. The pulse sequence also includes a series of fast spin echo readout excitation pulses applicable to the at least a number of slices in the slab of slices after an inversion time. Preferably, the aforementioned inversion time is selected so that blood in the slab is at or near the null point.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.